1. Field of the Invention
This invention relates to low density aluminum-lithium based metal matrix composites, and more particularly to a heat treatment for producing an aluminum-lithium composite with high hardness, strength, modulus and ductility.
2. Description of the Prior Art
An aluminum-lithium based composite generally comprises two components--an aluminum-lithium alloy matrix and a hard reinforcing second phase. The composite typically exhibits at least one characteristic reflective of each component. Ideally, an aluminum-lithium alloy matrix offers the low density, moderate ductility and high specific mechanical properties, and the high elastic modulus and abrasion resistance of the reinforcing phase.
Several aluminum-lithium based metal matrix composites prepared by ingot metallurgy techniques, e.g. melt infiltration, have been reported. In addition, there has been disclosed the preparation of aluminum-lithium based alloy systems by mechanical alloying. Such mechanical alloying techniques have been described by U.S. Pat. No. 4,594,222 as being suitable for the fabrication of aluminum-lithium based alloy powder. The technique therein taught involves the repeated fracturing and rewelding of a mixture of powder particles during high energy impact milling in a grinding or ball mill. A necessaryy prerequisite for the milling operation is said to be the presence of an organic-base processing control aid.
In some instances, mechanical alloying systems have incorporated, as reinforcing material, low volume fractions of carbides and oxide/hydroxides into the aluminum-lithium based alloy. The alloy is then hardened using heat treatments conventionally employed with monolithic aluminum-lithium base alloys, that is, heat treatments comprising the steps of solutionizing at temperatures above the solvus temperature followed by age hardening at temperatures below the solvus temperature. Such heat treatment procedures are often times complicated by the reinforcing material. When present in the aluminum alloy matrix, such reinforcing material significantly alters the aging response of the alloy. As a result, aluminum-lithium based composites have heretofore exhibited lower values of hardness, strength and ductility than those considered desirable for commercial applications.